The present invention relates to a sample analysis apparatus using electron beam irradiation, and more particularly to such an apparatus adapted for the case where there is a need that an electron gun chamber containing an electron gun to emit an electron beam must be evacuated to a higher degree of vacuum than a sample chamber containing a sample irradiated by the electron beam.
Examples of sample analysis apparatus using electron beam irradiation are a scanning electron microscope, an electron-probe X-ray microanalyser, etc. The principle of operation of these apparatuses is such that a sample to be analysed is irradiated by an electron beam whereby the information obtained from the sample and characteristic of the sample is detected. Here, the information obtained is in the form of transmitted electrons, reflected electrons, secondary electrons, absorbed electrons, Auger electrons, X-rays, cathode luminescence, etc. The means for detecting such information is one for converting the information into electric signals.
It is well known that when an electric field of about 10.sup.7 V/cm is concentrated on the pointed end of a metal rod, having a radius of less than 1000 A, electrons are emitted from the pointed end of the cold metal rod. The current due to this field emission increases with the increase in the intensity of the electric field concentrated on the pointed end. The electron current density due to the field emission is 10.sup.3 times higher than the corresponding electron current density due to thermionic emission. For this reason, the field emission source is called a high intensity electron generating source. Moreover, a practical source of electrons in thermionic emission has a diameter of several microns to several tens of microns while the diameter of a practical electron source in field emission is smaller than 100 A.
The most difficult problem that has prevented the electron gun of field emission type from being used in a sample analysis apparatus using electron beam irradiation, notwithstanding the above mentioned merits, is a difficulty in obtaining a high degree of vacuum in the order of 10.sup.-.sup.10 Torr which is necessary for generating a stable field emission current. With the recent progress of vacuum techniques, a high degree of vacuum in the order of 10.sup.-.sup.10 Torr has come to be obtained without much difficulty by the use of an ion pump. Accordingly, some attempts have been made to use an electron gun of field emission type in a sample anaylsis apparatus using electron beam irradiation.
The sample to be irradiated by electron beam is also placed in vacuum. The degree of vacuum in the sample chamber need not be so high as that in the electron gun chamber but has only to be 10.sup.-.sup.5 Torr in practice. In an actual apparatus, however, the electron gun chamber and the sample chamber must communicate with each other through a small aperture through which the electron beam travels. Therefore, if the sample chamber is merely kept at vacuum in the order of 10.sup.-.sup.5 Torr, a flow of gas from the sample chamber to the electron chamber takes place. This prevents a stable field emission from being established. In order to solve this difficulty, the sample chamber should be preferably kept at vacuum in the order of 10.sup.-.sup.7 -10.sup.-.sup.8 Torr.
As well known, the sample whose analysis has been completed must be replaced by another sample to be newly analysed. As methods of exchanging samples, there may be considered a method in which the sample chamber is separated from the electron gun chamber in vacuum continuum, the sample chamber is exposed to the atmosphere, a new sample is substituted for the old one, the sample chamber is evacuated up to 10.sup.-.sup.7 -10.sup.-.sup.8 Torr and the sample chamber and the electron gun chamber are brought into vacuum communication with each other (this method is hereafter referred to simply as the first method). However, this method has a drawback that the sample chamber must be evacuated up to a rather high degree of vacuum, thereby requiring a relatively long time for reevacuation.
There may be considered another method of exchanging samples, according to which a sample exchange chamber capable of being connected with and disconnected from the sample chamber in vacuum continuum is provided (this method is hereafter referred to simply as the second method). The sample exchange chamber has only to have a volume enough for sample exchange and usually has a smaller volume than the sample chamber. In the sample exchange operation, the sample is transferred from the sample chamber to the sample exchange chamber kept at vacuum in the order of 10.sup.-.sup.3 -10.sup.-.sup.5 Torr. The sample exchange chamber is then separated from the sample chamber and replenished with air. The old sample is replaced by a new one and the sample exchange chamber is again evacuated up to 10.sup.-.sup.3 -10.sup.-.sup.5 Torr. Then, the new sample is transferred from the sample exchange chamber to the sample chamber after the sample exchange chamber has been brought into vacuum communication with the sample chamber. According to this second method, such a problem as raised in the first method mentioned above can be solved, but there is still left a problem that a complicated and expensive mechanism to transfer the sample from the sample chamber to the sample exchange chamber or conversely in vacuum is needed. Moreover, when the detector to detect the information obtained from the sample is replaced by another detector in this second method, the vacuum condition of the sample chamber kept at 10.sup.-.sup.7 -10.sup.-.sup.8 Torr must be destroyed. This is the same problem as is caused in case of sample exchange in the first method.